1. Field of the Invention
This invention relates to a cryogenic fluid storage system and, more specifically, to a cryogenic fluid storage system having a check valve on the vapor line at a location between the vapor space and the use device.
2. Background Information
Cryogenic liquids, such as liquid natural gas (LNG), nitrogen, oxygen, CO2, hydrogen and the like, are substances that normally exist as gasses, but are liquids at cold temperatures. Special vessels and systems must be used to store and transfer cryogenic liquids because of difficulty in maintaining the extremely cold temperatures. Such vessels typically include a double walled vessel having a vacuum in the annular space. While the vacuum provides an effective insulation, the insulation is not perfect and, as such, heat penetrates the vessel. When heat is added to the cryogenic liquid, a portion of the liquid returns to the gaseous state. The gas within the vessel increases the internal pressure. The build up in pressure may even occur when liquid is being removed from the vessel and being delivered to a use device, such as an engine. Eventually, to prevent over pressurization of the vessel, the gas must be removed from the vessel. It is desirable, however, to not waste the gas by venting the gas to the atmosphere. That is, if possible, it is desirable to use the vented gas in the use device.
For example, where the cryogenic liquid is LNG, the use device is typically an engine. The following description shall use the example of LNG and an engine, but it is understood that the system described herein is applicable to any cryogenic liquid and any use device. The fuel system for the engine includes the cryogenic vessel, a delivery line extending from the cryogenic vessel to the engine, a vaporizer on the delivery line and an economizer circuit. Within the cryogenic liquid vessel are a liquid space and a gas, or vapor, space. The delivery line includes a portion adjacent to the vessel that, in normal operation, contains the cryogenic liquid, hereinafter, the “liquid line.” The liquid line is in fluid communication with the cryogenic vessel liquid space. The economizer circuit, or vapor line, is in fluid communication with both the cryogenic vessel vapor space and the liquid line. Down stream of this joint between the liquid line and the vapor line, the delivery line may contain liquid, gas, or a combination of liquid and gas. Because the engine uses the natural gas in a gaseous state, a vaporizer may be located upstream of the engine.
In normal operation, if the vessel does not have a sufficient pressure, a small quantity of cryogenic liquid may be removed from the liquid space, passed through a vaporizer where it is converted to gas, and returned to the vapor space of the cryogenic liquid vessel. Alternatively, when the engine is not running, any excess cryogenic liquid from within the delivery line is allowed to evaporate and is returned to the vapor space through the vapor line of the economizer circuit. This gas pressurizes the cryogenic liquid vessel so that, when the engine is running, the pressure within the vessel causes the cryogenic liquid to exit the vessel to be delivered to the engine. Once the vessel is pressurized the delivery line may be opened to deliver LNG to the vaporizer or engine. Within the vessel, the vapor and the cryogenic liquid are at the same pressure. However, due to the additional pressure created by the weight of the cryogenic liquid, there is a slightly higher pressure acting on the liquid line. Thus, the path of least resistance to fluid flow is through the liquid portion of the delivery line and, when both the economizer circuit and the delivery line are open, fluid will flow from the liquid space within the vessel. Alternatively, the economizer circuit vapor line may include a regulator structured to close when the pressure in the vessel is below a set limit. This ensures that the liquid line is the path of least resistance.
As noted above, heat causes the cryogenic liquid within the cryogenic liquid vessel to be converted to gas and may cause an undesired increase in pressure. That is, the vessel may become over-pressurized. In this situation, gas must be removed from the vapor space to prevent a catastrophic failure of the vessel. One method of removing gas is to simply vent the gas to the atmosphere. This, of course, results in wasted gas. To prevent the venting of gas to the atmosphere when the cryogenic vessel is over-pressurized, gas may be removed from the vapor space within the cryogenic vessel and delivered to the engine. While a direct connection between the cryogenic vessel vapor space and the engine is possible, more typically, the gas is withdrawn through the economizer circuit. That is, because the economizer circuit is in fluid communication with the delivery line, high pressure gas may be transferred through the vapor line to the delivery line and then to the end use. Thus, when the pressure within the vessel exceeds a set limit, the regulator on the economizer circuit vapor line opens allowing gas to flow from the vapor space to the delivery line. However, because the pressure in the vapor line and the liquid space is, essentially, equal, and because the vapor line is also connected to the liquid line, there must be a device on the liquid line to increase the back pressure so that the vapor line is the path of least resistance for the fluid flow. Typically, flow of the cryogenic liquid within the liquid line is reduced by a pressure relief valve, or a restricted orifice, structured to create a back pressure in the liquid line. This additional back pressure ensures that, when the economizer circuit vapor line regulator is open, the vapor line is the path of least resistance and gas within the vapor space is expelled through the economizer circuit to be delivered to the engine, thereby reducing the pressure within the cryogenic vessel. See, e.g., U.S. Pat. No. 5,421,161.
The disadvantage of this system is that the flow of cryogenic liquid is not stopped when the regulator is open. That is, even in an over-pressurization situation, the pressure relief valve or a restricted orifice allows liquid to move through the delivery line. Because the use device is receiving fluid from both the vessel liquid space and vapor space, the speed of pressure reduction is slower than if the use device was receiving fluid from the vapor space only.
There is, therefore a need for a cryogenic liquid storage vessel structured to rapidly reduce the internal pressure while delivering the gas from the vapor space to the use device.
There is a further need for a cryogenic liquid storage vessel structured to rapidly reduce the internal pressure in the cryogenic liquid storage vessel that stops the flow of cryogenic liquid through the delivery line while gas from the vapor space is being delivered to the use device.